Electrochemical elements such as a lithium ion secondary battery and an electric double layer capacitor have advantageous characteristics that they are small, lightweight, their energy density is high, and they can be repeatedly charged and discharged, and accordingly their demand is expanding rapidly. The lithium ion secondary battery has a relatively large energy density, and it is used in the fields such as cellular phones and notebook personal computers, meanwhile the electric double layer capacitor can be rapidly charged and discharged, and it is used as a memory backup small power source in personal computers and the like. Furthermore, the electric double layer capacitor is expected to be applied as a large sized power source for electric vehicles. Moreover, the redox capacitor using the oxidation-reduction reaction (pseudo electricity double layer capacitance) on the surface of a metal oxide or a conductive polymer also attracts attention because of the size of its capacitance. As for these electrochemical elements, along with the expansion of their applications, further more improvements are required for lower resistance, higher capacitance, more excellent mechanical properties and the like. Under such circumstances, in order to enhance the performances of electrochemical elements, various improvements are also made in the materials which form electrochemical element electrodes.
The electrochemical element electrodes are, in general, made by laminating active material layers which are formed by bonding electrode active materials such as activated carbon and lithium metal oxide, and electric conductive materials on a collector.
Patent Documents 1 and 2 disclose a method of pressurizing and forming composite particles obtained by bonding particulate electrode active materials and particulate electric conducting auxiliary agents by a binder to obtain active material layers. The composite particles used in the Patent Documents 1 and 2 have the structure where particulate electrode active materials and particulate electric conducting auxiliary agents are distributed uniformly in the composite particles as shown in FIG. 1. However, the composite particles have inferior formability, and accordingly, it has been difficult to obtain electrode sheets stably and continuously.
Further, Patent Documents 3 discloses a method where a slurry mixed material containing electrode active materials, thermosetting resin, and solvent is formed, and this mixed material is granulated by the spray dry method to obtained composite particles, and the composite particles are fixed by the hot pressing, roll pressing, or other means on a collector to form active material layers. The particles obtained in the Patent Documents 3 are, as shown in FIG. 2, hollow particles having the husks formed with bonded particulate electrode active materials.
However, in the electrodes formed of these particles, the density of its active material layers is low, and accordingly, only electrochemical elements with small capacitance have been obtained.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-78943    [Patent Document 2] United States Patent Publication No. 2005/0064069    [Patent Document 3] Japanese Unexamined Patent Application Publication No. H09-289142